The invention relates in general to the coking of coal and in particular to a new and useful process for releasing the ammonia bound in coal water, the coal water being used, in ammonia-hydrogen sulfide cycle washers provided with deacidification and ammonia expulsion, for crude coking plant gas so as to wash out the ammonia. The acid components remaining in the coking plant gas after prewashing are retreated with alkali solution in an afterwasher, and the alkali solution enriched with acid components are drawn off from the alkali washing and used for releasing ammonia bound in the coal water and replaced by fresh alkali solution.
Known is a process for expelling bound ammonia from aqueous liquids, in particular from the aqueous condensates of low-temperature carbonization and coking of coal and lignite, bituminous shale and peat, as well as from the effluents of the processing of low-temperature carbonization or coking gases, in which the ammonia is bound to slightly volatile, strong acids, the aqueous liquids being mixed with aqueous solutions or suspensions of sulfidic alkaline substances which had served for the absorption of hydrogen sulfide from gases of low-temperature carbonization or coking. The mixture is subsequently heated when, as aqueous alkaline solutions are used, those sodium, potassium and lithium waste liquors which had been used for H.sub.2 S scrubbing prepurified coke or low-temperature carbonization gases. In particular, in this process uses waste liquors which were employed in a hydrogen sulfide after-washery following the ammonia-hydrogen sulfide cycle washery. (DE OS No. 2,537,640).
A process is also known for expelling fixed ammonia from gas liquors where the service water leaving the ammonia scrubber is combined in the installation for expelling the ammonia with the cold stream of the wash liquid branched off from the cycle of the hydrogen sulfide scrubber, fresh soda or potash lye being continuously supplied to the wash liquid cycle of the hydrogen sulfide water and saturated washing agent liquid removed, using as starting solution in the hdyrogen sulfide scrubber a 3.0 to 5.0% preferably 3.5% soda lye, whose addition to the hydrogen sulfide scrubber can be regulated via a valve that through a colorimeter installed in the effluent stream of the ammonia expeller. A partial stream of the wash liquid cycle is so controlled as to quantity that all fixed ammonium salts are decomposed (DE OS No.2,734,495). Here the sodium sulfide-containing partial stream of the service water taken out of the ammonia scrubber, or from the gas cooler, can be added even before the ammonia expeller but preferably before the suction pump.
Further a process for expelling bound ammonia from ammonia-water in an ammonia expeller column is known, where a sodium sulfide liquor is added to the ammonia-water as strong base (DE PS No. 2,412,026).
Lastly, a process for the purification of coke oven gas is known, where from the detarred and cooled crude gas the ammonia is washed out with water, possibly with addition of sulfuric acid, and the hydrogen sulfide is washed out through a scrubbing cycle containing sodium or potassium carbonate, whence the hydrogen sulfide is expelled by means of carbon dioxide and to which certain quantities of soda or potash lye are continuously supplied and corresponding quantities of wash liquid removed, and wherein the wash liquid removed from the cycle of the hydrogen sulfide scrubber is admixed to the waste water of the ammonia scrubber for expulsion of the fixed ammonia present therein (DE AS No. 2,585,959).
In the processes known until now, the quantity of alkali necessary for the release of the bound ammonia contained in the coal liquors or, respectively, in the condensates, is used in a preceding scrubbing process to wash out the hydrogen sulfide as well as other acid components from the crude coke oven gas. Thereafter it is sent to the ammonia expeller or fed into the inflow line to the expeller, where it converts, the so-called fixed ammonium salts, and liberates the ammonia bound therein. For the complete conversion of the ammonium salts of the coal water one needs, as experience has shown, an excess of these desulfurization liquors. During the preceding desulfurization there is absorbed from the coke oven gas by the alkali solution besides hydrogen sulfide also a considerable proportion of hydrogen cyanide and carbon dioxide. Now if, in the expeller, an excess of this spent alkali liquor is added, the complete conversion and expulsion of the ammonia from the ammonium salts can indeed be achieved. Besides the carbon dioxide, however, there remains a considerable proportion of harmful hydrogen cyanide and also hydrogen sulfide bound to alkali in the waste water, thereby impairing its quality, so that herein an improvement is strived for.
In addition, one strives for an improvement of the hydrogen sulfide extraction in the known ammonia-hydrogen sulfide scrubber cycle, so that after the alkali after-scrubber a low hydrogen sulfide end content is obtained in the purified gas. Such an increase of the degree of extraction of hydrogen sulfide would be possible if one could increase the ammonia supply in the hydrogen sulfide scrubber of the ammonia-hydrogen sulfide scrubber cycle. This, however, should be effected without substantial extra expenditure for materials and energy, so as not to burden the profitability of the process.